Diesel fuels and/or biodiesel fuels typically contain wax, and when subjected to low temperatures, these fuels often undergo wax crystallization, gelling and/or viscosity increase. This reduces the ability of the fuel to flow and creates filter plugging which adversely affects the operability of vehicles using these fuels. Flow improvers have been used to modify the wax structure as it builds during cooling. These additives are typically used to keep the wax crystals small so that they can pass through fuel filters. Also, pour point dispersants are sometimes used in diesel fuel to ensure that it can be pumped at low temperatures.
Due to environmental concerns and the decline of known petroleum reserves with subsequent price increases of petroleum, biodiesel fuels are becoming a focus of intense research and development efforts. Biodiesel fuels typically comprise fatty acid esters, prepared for example by transesterifying triglycerides with lower alcohols, e.g. methanol or ethanol. A typical biodiesel fuel is the fatty acid ester of a natural oil (i.e. rapeseed oil or of soybean oil, as non-limiting examples). One of the major problems associated with the use of biodiesel is its poor cold flow properties resulting from crystallization of saturated fatty compounds in cold conditions, as indicated by its relatively high cloud points (CP) and pour points (PP). A 20° C. reduction in cold filter plugging point is necessary for some biodiesel fuels to find utility in colder climates such as those of North America and Europe in winter.
Several efforts to mitigate the low-temperature problems of biodiesel have been investigated over the past several years. Many popular approaches have included blending biodiesel with conventional diesel fuel, winterization, and use of synthetic additives. Also, studies have been performed to show the diversification in the feedstock and genetic modification of the feedstock, aimed to provide a reduction in the saturated content of the fatty acid methyl esters (FAME) in biodiesel as well as modification of FAME composition/profile of the fuels. While there have been efforts to create additives that may reduce the PP and cold filter plugging point (CFPP) of fuels, many are not cost effective. Also, increasing the unsaturated content of biodiesel may improve its cold flow properties, but also leads to the alteration of the oxidative stability of the fuel. The overall thermal behavior of biodiesel is affected by the relative concentration of its saturated and unsaturated FAME components. The cold flow issue is primarily a multifaceted problem of crystallization (of saturated FAMEs) in solution (unsaturated FAMEs) which can be approached from several angles.
Several approaches have been utilized to lower the onset temperature of crystallization of biodiesel, targeting particularly the saturated FAMEs such as methyl palmitate (MeP) and methyl stearate (MeS), which influence most its flow behavior at low temperature. The most popular approach is the use of crystallization depressant additives.
Saturated triacylglycerols (TAGs) and dimers of TAGs, particularly those having two double bonds at the sn-1 and sn-3 positions, have been found to be effective in suppressing the crystallization of FAMEs. It has been shown for example that FAMEs, such as MeS and MeP, and TAGs, such as 1,3-dioleoyl-2-palmitoyl glycerol (OPO) and 1,3-dioleoyl-2-stearoyl glycerol (OSO), form eutectic as well as peritectic systems through more or less loosely bound stoichiometric compounds. The eutectic temperatures induced by the TAGs were much lower than the melting points of both pure compounds.
In order to suppress the crystallization, an additive needs to concurrently have a structural similarity with the crystallizing substances in order to favor the required interaction, and features that would suppress the formation of organized structures. In the case of di-cis-unsaturated TAG molecules (such as OPO, OSO), we have found that the mechanism for disruption of crystallization is dependent on the peculiar geometry of the TAG: the “straight” acid chain promotes the interaction with the FAME (MeS, MeP) and participates easily in the lamellar packing of the equally “straight” FAME, while its two kinked unsaturated oleic acid chains effectively halts additional saturated FAMEs from participating in the packing due to steric hindrances. The interaction of the relatively large glycerol group of the TAG with the FAME molecules could be repulsive, adding to the suppression of the crystallization effect. This is realistic since the crystallization behavior of TAGs and FAMEs, and more generally of oils and fats, is directly related to structural details such as the length of the acyl chains, degree of unsaturation and conformation of the glycerol groups.